1. Technical Field
The present invention relates to methods and transmitter devices for transmitting a modulated signal in a transmission system. In particular, the present invention relates to improvements for transmission spectra of shift keying signals, such as Gaussian Minimum Shift Keying (GMSK) or Enhanced Data rates for GSM Evolution (EDGE) signals.
2. Description of the Related Art
The GSM mobile communication standard is now in place throughout most areas of the world. Developed in mid-1980, it is a mature communication technology and provides reliable service. GSM systems operate using a Gaussian Minimum Shift Keying (GMSK) modulation scheme that has a constant envelope and no AM (Amplitude Modulation). Despite of new 3G (3rd Generation) wireless developments, efforts have still been employed to bring data capabilities to existing wireless handset and base station designs. This move-forward mentality sparked the development of so-called 2.5G technologies that would allow wireless operators to deliver voice and higher-speed data services (up to 384 Kbps) without having to rip out the entire infrastructure.
In order to provide more bandwidth-efficient signals for high data rate applications, the EDGE signaling format has been developed as an upgrade for current GSM architectures.
The EDGE format has strong AM. Complex Dirac impulses that are organized as an 8-ary MPSK (M-ary Phase Shift Keying) may occur at the input of the required pulse shaping filter. EDGE can be implemented as a linear in-phase (I) and quadrature-phase (Q) modulation. I and Q are Cartesian components of a complex modulation signal. The EDGE signaling format was designed so that spectral and other characteristics would be compatible with and suitable for overlaying on existing GSM and TDMA (Time Division Multiple Access) systems employing GMSK. Whereas the I- and Q-modulation in EDGE is a linear function of the input coefficients, the continuous phase modulation (CPM) of GMSK has a nonlinear response at I and Q. Different types of precoding and interleaving of transmitted bits are implemented in GSM and EDGE systems. The EDGE signal is constructed by mapping incoming bits into 8-ary exciting Dirac impulses, which correspond to points in an ordinary 8-level phase-shift keying (8PSK) signal constellation. Cartesian components of these signal points are taken as I&Q signals, which are input to the pulse shaping filter and corresponding rails in an I&Q Quadrature Modulator. Or the instantaneous Modulation-Frequency and Modulation-Amplitude is derived from it for a polar modulation in a Polar Modulator instead.
An additional step in the EDGE modulation is a rotation of ⅜p for succeeding symbols. Transitions between symbols occur gradually in EDGE and GMSK signals, producing a compact spectrum. The spectral characteristics of the modulation schemes used in both systems are very similar. But the EDGE signal is not constrained to possess a constant RF amplitude. This signal format places stringent requirements on amplifiers in terms of linearity, contrary to the GMSK signal.
Like the GMSK signal employed in GSM, also the EDGE signal employs a modulating pulse with leading and trailing skirts, which extend into neighboring symbol intervals. Unless special care is taken, this type of pulse can cause successive received symbol statistics to interfere with each other at the receiver output. Such interference causes one symbol to interfere with the voltage level of some of its predecessors and successors. As it was used in older systems with some pulses called Nyquist pulses, it is possible to achieve independence between sampling points at receiver output signals even though the pulse extends for more than one symbol. These pulses allow trailing and leading skirts of successive pulses.
Newer systems than GSM and EDGE use highly complex algorithm in Digital Signal Processors in order to detect the signal.
In addition to the traditional RF approaches, DSP techniques have also emerged as a key method for performing filtering within modern receivers. Multi-path propagation of RF radio signals in a hilly terrain causes long Inter-Symbol-Interference (ISI). The signal processing employed in typical GSM and EDGE receiver designs often includes algorithms that deliver sophisticated equalization and coding schemes. The algorithms employed within these functions can process raw received symbols in an optimum fashion to actually improve end-to-end BER (bit error rate) performance.
The required transmission (Tx) signal for all GSM systems as GSM900, DCS1800, PCS1900 and US850 is 0.3GMSK, as specified in the specification for GSM/EDGE: 3GPP TS 45.005 V7.0.0 (2005-04). According to these recommendations, errors at the phase of GMSK must be below 20° peak and below 5° rms. There is no amplitude modulation (AM) at the Tx signal in order to keep the required Power Amplifier (PA) simple. The GMSK Tx spectrum is rather wide due to this rule. However, a receiver uses the spectrum only within a range of about +/−110 kHz.
Additionally, Tx signals for the 8PSK-system EDGE are also defined in the above recommendations. Test limits are defined at the Error Vector Magnitude (EVM). An average EVM should remain below 9% and peak errors below 30%. There are specified templates for the required limits of Tx spectra. The limit for 400 kHz separation is −60 dB for GSM and −56 dB for EDGE. These locations in the templates are named 400 kHz corners.
In both GMSK and EDGE cases, there is a little degree of freedom for system improvements. It is however not intended to create new standards for Tx error rules or the like.
Peter Jung, “Laurent's Representation of Binary Digital Continuous Phase Modulated Signals with Modulation Index 1/2 Revisited”, IEEE Transactions on Communications (ISSN 0090-6778), vol. 42, no. 2-4, pt. 1, p. 221-224, 02/1994, discloses that composition of GMSK signals is possible by nonlinear superposition of signals in a QAM system (Quadrature Amplitude Modulation). One of these pulse shaping functions is named C0. The same C0 is used in the specified standard for EDGE transmit modulation.